Wireless communications systems, such as “friend” or “friendly” wireless networking systems, are often power- and bandwidth-limited. These systems can be constrained due to safety regulations or size, weight, and power limitations. A typical network topology for these systems is easily penetrated by an “unfriendly” interloping jamming device. Typically, this jamming device issues a high level of signal interference for jamming any transceivers present in the “friendly” system.
In addition, one or more of these transceivers will typically not provide enough transmission power as a defense again the jamming device. Thus, intelligent jamming avoidance for the network is necessary rather than a higher transmission power with respect to the jamming device. For example, the jamming device can take advantage of these power-limited networks by finding and adapting to a “victim” signal as nearly as quickly as the victim signal changes frequencies. As a result, any attempts to avoid the jamming device (such as moving to another channel) are likely ineffective, unless the frequency switching speed is impractically short. In addition, since the jamming device quickly learns the “victim” signal, the bandwidth available will need to be utilized wisely. For example, typical parameters that the jamming device learns by listening include center frequency, bandwidth, modulation type, hopping scheme, hopping frame timing, and signal transmission burst size(s). In the case of most commercial and industrial communications, these parameters are readily discernable. For example, users of the jamming device may already know the entire channel allocation, bandwidths, and modulation types from earlier site surveys and studies.
The jamming device will surely not know the unpredictable random progression of frequency hopping through the channels in a time-frequency domain (assuming any scrambling algorithm of the jamming device is made adequately robust). In short range applications such as inside buildings, industrial campuses, and within aircraft however, any propagation delay experienced by this random progression, which might otherwise corrupt the jamming device's estimates of hop framing, is often too insignificant to benefit any of the “friend” communication links. As a result, current jamming devices are able to “look-through” their own jamming to see the signal transmission, determining this random progression of frequency hopping through the channels by observing the beginning of each dwell, and jamming the reminder before the dwell time of the signal transmission is over, successfully jamming the transmission. This “look-through” class of jamming devices includes: (1) Repeat Jammers (for example, a digital radio frequency memory, or DRFM-enabled class of jamming devices); (2) Follower Jammers (copying any frequency hopping); and (3) Matched Spectrum Jammers, among others.
Each of the above-mentioned jamming devices employs current off-the-shelf technology (and therefore is easily implemented), and is a high probability threat. Typical adaptations against these jamming device classes involve a temporary loss of data connectivity, which is intolerable in nearly all commercial and industrial wireless networking applications.